Single pole double throw (SPDT) switches for mobile handsets are typically located between an antenna and a radio front-end. Generally, one switch is located between a transmitter block (TX) output and an antenna; the other is located between a receiver block (RX) input and an antenna. Both switches never close simultaneously to prevent a direct path from TX to RX. The switches are required to have small insertion loss for minimum power loss in transmit mode, and noise figure and large isolation for minimum leakage in receive mode. There exist trade-offs between small insertion loss and large isolation. The size of transistor for a switch should be large to achieve small insertion loss with a multiple parallel configuration, but a large transistor brings parallel large parasitic capacitance and prevents the achievement of large isolation.
High Electron Mobility Transistor (HEMT) technology is currently preferred for implementing a SPDT switch because of its best performance for switch operation due to high breakdown voltage and small parasitic capacitance. However, the technology is one of the most expensive technologies. Moreover, some applications require high power handling capability at the TX switch. In that case, series switches are cascaded to overcome high voltage swing from the high power but high insertion loss cannot be avoided.